Production of ventless delay initiators for explosive or other charges capable of exothermic reaction



952 o. LEWIS ET AL 2,619,035

PRODUCTION OF VENTLESS DELAY INITIATORS FOR EXPLOSIVE oR OTHER CHARGESCAPABLE OF EXOTHERMIC REACTION Filed Nov. 1, 194a lnvenlorzs- [a'walOwerzlewz's id 13 RoberlDavid Jo/z Owens A Home Patented Nov. 25 1952Units RRODUCTION OF VENTLESS DELAY INI- TIATORS FOR EXPLOSIVE OR OTHERCHARGES CAPABLE OF EXOTHERIWIC REACTION tion of Great BritainApplication November 1, 1948, Serial No. 57,776 In Great BritainDecember 31, 1947 3 Claims.

The present invention relates to improvements in the production ofventless delay electric initiators, that is to say devices for effectingthe excitation of the required reaction in explosive or other chargescapable of exothermic reaction, these initiators being of the kindwherein an electrically actuated igniting charge, an initiating chargeand an interposed delay fuse element are enclosed in a ventlesscontainer and the said igniting charge and delay fuse element produce aninsufficient amount of gases or vapours to break the closure of thecontainer before the ignition of the initiating charge has beeneffected. Amongst the advantages of this kind of delay electricinitiator are the ease with which moisture can be excluded from itsinterior, and the fact that no incandescent products or flames areejected while the igniting charge and the delay fuse are being consumed,which latter advantage is often of special importance in the case of thekind of delay electric initiators required for detonating explosives, asin blasting operations. In this kind of delay electric initiator, namelya delay electric detonator, the initiating charge comprises a detonatingexplosive, and ventless delay electric detonators have found extensiveapplication in blasting operations where it is required that explosivecharges connected up in a single circuit should detonate successivelywith predetermined intervals between the detonations.

It is an object of the present invention to provide new or improvedventless delay electric initiators, for example ventless delay electricdetonators, whose production can be more expeditiously, economically orconveniently effected than that of the known ventless electricinitiators.

Hitherto the delay fuse elements employed in commercial delay electricinitiators have usually comprised each a length of rigid metal tubinginto which the powdered delay composition that yields little or no gasor vapour as it is consumed is introduced in increments, each of whichincrements is highly compressed into said length of tubing before thenext increment is introduced. For a given composition the number ofpressings required depends mainly on the length of the column of thecompressed composition required to provide the required delay interval.This incremental loading has hitherto been regarded as essential inorder to give the composition the necessary mechanical strength and toensure that the packing should be to a uniform density all along thelength of the column, so that a rigid tube of a given length loaded withthe same composition will always give the same delay period. The rigidmetal tube so charged with the gasless delay composition is pushed intothe tubular container of the initiator, in which it must form a tightfit so that no annular space is left by which flame can travel directlyfrom the igniting charge to the initiating charge, and so that it willbe secured from sliding motion in the free space.

It would be desirable to increase the rate at which ventless delaydetonators can be produced, and especially desirable to increase therate of production of delay fuse elements, hitherto requiring to beloaded in a multiplicity of increments.

According to the present invention a ventless delay electric initiatoris characterised in that the delay fuse element thereof consists of aflexible envelope surrounding an unbroken continuously extruded core ofa thermoplastic composition of low gas evolution characteristicscomprising inorganic reducing agent and oxidising agent powders adaptedon ignition to react exothermically with the formation of non-volatileproducts accompanied by at most insignificant amounts of volatileproducts, said reducing agent being present in at least thestoichiometric proportion, embedded in a continuum of an organicthermoplastic.

In accordance with a preferred embodiment of the present invention aventless electric initiator has a delay fuse element cut off from alength of a fuse having a flexible envelope surrounding an unbrokencontinuously extruded core of a thermoplastic composition of low gasevolution characteristics comprising inorganic reducing agent andoxidising agent powders adapted on ignition to react cxothermically withthe formation of non-volatile products accompanied by at mostinsignificant amounts of volatile products, said reducing agent beingpresent in at least the stoichiometric proportion, embedded in acontinuum of an organic thermoplastic.

The core may if desired be extruded around a filament, the nature ofwhich will have an effect on the linear burning speed of the delay fuse.

In accordance with a further preferred embodiment of the invention thedelay fuse element is secured in. position by pressing a perforatedinverted metal cup over the end thereof remote from the initiatingcomposition, instead of relying on a friction fit between the textileenvelope and the wall of the tubular container. The use of such a cupalso prevents the textile envelope from being ignited by the electricfusehead or the like and is especially convenient when an additionaligniting composition for the thermoplastic core composition is present.The organic thermoplastic may be one of low infiammability, for instancea thermoplastic based on a polymerised vinyl, acrylic or methacrylicderivative or a cellulose organic ester or cellulose ether, and theoxidising and reducing agents may advantageously comprise red lead andsilicon.

The composition of the extruded core is preferably so adjusted that nomatter how short the delay time required the length of the fuse elementcut off will exceed its diameter.

In inserting the delay fuse elements used according to the presentinvention into the tubular containers, the soft and somewhatcompressible nature of the textile envelope renders the attainment ofthe necessary tight fit easy without the application of loadingpressures so high that the core shows any tendency to crumble in thecourse of the operation. An expansion space must be left between the endof the delay fuse element and the closure. If desired or necessary asmall charge of supplementary preferably gasless igniting compositionmay be interposed between the end of the delay fuse element and thecharacteristic charge of the initiator. It is also sometimes desirableto press a small charge of a gasless igniting composition directly ontop of the delay fuse element in order to ensure its ignition from theelectrically ignitable charge, which will usually be spaced from it. Theelectrically ignitable charge may take the form o an electric fusehead.

Since the delay period is controlled by the length of delay fuse elementcut off from the greater length, delay electric initiators of relativelylong delay characteristics can be produced as quickly as those ofrelatively short delay characteristics according to the invention, andthe cutting of a length of plastic core fuse is a more expeditiousoperation than the pressing of even a single charge into a rigid metaltube.

The invention is further illustrated by the diagrammatic drawingaccompanying the specification, which represents a vertical axialsection of one form of delay electric initiator, more particularly adelay electric detonator, in accordance with the invention.

In the drawing 1 is an aluminum detonator tube, 2 is a detonatingexplosive charge of tetryl compresssed into the end of the tube and 3 isa more sensitive detonating explosive charge compressed on top of thecharge 2 and consisting essentially of a mixture of lead azide, and leadstyphnate, 5 is the envelope of waterproof coated textiles, and 0 is thethermoplastic core composition of a delay fuse element cut off from alength of delay fuse having a textile envelope surrounding an unbrokencontinuously extruded core of for example a 75:25 mixture of red lead,silicon and a plastic consisting of methyl methacrylate polymer anddibutyl phthalate in equal proportions, the proportion of the activeingredients to the plastic being 90:10 all these proportions being byweight. The diameter of the core is approximately 2 millimetres. I2 is a36 S. W. G. central copper wire filament around which the thermoplasticcore composition 6 has been extruded. i is a layer of a gasless ignitingcomposition consisting for instance of antimony and potassiumpermanganate in the ratio 30:70 by weight. 1 is a layer of the samecomposition as composition 4, and 8 is a perforated inverted metal cuppressed over the composition 1 and over the top end of the delay fuseenvelope 5 so as to secure the delay fuse element longitudinally in thetube and effectually seal off the space above this cup from thecomposition 4. 9 is an electric fusehead, ll and H are insulatedelectric leading wires and I9 is an axially perforated sealing plug ofrubber like material crimped between the metal tube 1 and the wires IIand H as shown at [3, whereby there is attained a seal capable ofwithstanding an internal pressure greater than the maximum pressure thatcan be produced up to and including the time required for combustion ofthe intermediate charge 4. The space it between the end of the plug l0and the inverted cup 8 and hence the length of the tube I is accordinglysufficient to ensure that the hot pressure developed in the initiator bythe successive consumption of the compositions of the electric fusehead9, the igniting charge I, the core composition 5 of the delay fuseelement together with any gases that may arise from charring of itsenvelope 5, and the igniting charge 4 will not unseal it. The length ofthe aforesaid delay fuse element required is about 0.7 centimetre persecond of delay period.

We claim:

1. A delay electric detonator comprising an electrically actuatedigniting charge, an initiating charge, and an interposed delay fuseelement consisting of a length cut off from a fuse comprising a flexibleenvelope surrounding a charge based on inorganic reducing agent andoxidizing agent powders enclosed in a detonator tube, said flexibleenvelope surrounding an unbroken continuously extruded core of athermoplastic composition of low gas evolution characteristicscomprising inorganic reducing agent and oxidizing agent powders embeddedin a continuum of an organic thermoplastic and adapted on ignition toreact exothermically with the formation of non-volatile productsaccompanied by at most insignificant amounts of volatile products, saidreducing agent being in at least the stoichiometric proportion, and saidthermoplastic being of low inflammability and selected from the groupconsisting of vinyl, acrylic, methacrylic, cellulose organic esters andcellulose ether polymers, so as to form a ventless delay initiator.

2. A ventless delay electric initiator as set forth in claim 1 whereinthe said core of thermoplastic composition of low gas evolutioncharacteristics includes a central filament.

3. A ventless delay electric initiator as set forth in claim 1 whereinthe oxidizing and reducing agent powders comprise red lead and silicon.

IDVVAL O. LEWIS. ROBERT D. J. OWENS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,825,224 Ellsworth Sept. 29,1931 2,139,581 Hanley Dec. 6, 1938 2,410,801 Audrieth NOV. 12, 1946FOREIGN PATENTS Number Country Date 580,409 Great Britain Sept. 6, 1946

1. A DELAY ELECTRIC DETONATOR COMPRISING AN ELECTRICALLY ACTUATEDIGNITING CHARGE, AN INITIATING CHARGE, AND AN INTERPOSED DELAY FUSEELEMENT CONSISTING OF A LENGTH CUT OFF FROM A FUSE COMPRISING A FLEXIBLEENVELOPE SURROUNDING A CHARGE BASED ON INORGANIC REDUCING AGENT ANDOXIDIZING AGENT POWDERS ENCLOSED IN A DETONATOR TUBE, SAID FLEXIBLEENVELOPE SURROUNDING AN UNBROKEN CONTINUOUSLY EXTRUDED CORE OF ATHERMOPLASTIC COMPOSITION OF LOW GAS EVOLUTION CHARACTERISTICSCOMPRISING INORGANIC REDUCING AGENT AND OXIDIZING AGENT POWDERS EMBEDDEDIN A CONTINUUM OF AN ORGANIC THERMOPLASTIC AND ADAPTED ON IGNITION TOREACT